a. Field of the Invention.
This invention relates generally to the field of radio communications, and more specifically to radio receivers having a variable bandwidth to optimize reception of input signals having widely varying data rates.
B. Description of the Prior Art.
Radio receivers of the heterodyne and superheterodyne type have been a standard of the radio receiver industry for many years. In general, such receivers are designed by determining the maximum bandwidth which will be required for the anticipated input signals, building a bandpass filter capable of passing that bandwidth, and adjusting the intermediate frequency to the center frequency of the filter. Once the receiver is designed and constructed, the intermediate frequency must be held constant as must the width of the passband of the filters. Obviously, the receiver so designed is optimized only for a single, specific input data rate, that rate being the highest rate which the receiver is designed to receive. For any lower input data rate, the bandwidth will be greater than the optimum, with a resulting decrease in reception quality, or a decreased signal-to-noise ratio. Efforts at solving the problem of maintaining optimum quality reception as the input data rate varies are typified by U.S. Pat. No. 3,551,857 to Langer, in which is disclosed a frequency selective system capable of passing signals in more than one frequency range. The solution utilized therein includes a plurality of discrete filters capable of being switched in and out by an operator, whereby it is possible to select that filter which is most near the optimum filter for a given input signal.
For many applications, it is desirable to be able to select, on a continuous rather than a discrete basis, a filter whose bandwidth is the optimum for a given input signal data rate. It is to this end that the present invention is directed.